The present invention relates to a system and method of introducing a sample for analytical atomic spectrometry. It finds application in particular in the analysis of products in liquid form, in solution, or in suspension in a liquid, such as stones, earth and sediment, drinking waters, residual waters, oils, foods and biological and metallurgical samples.
Analytical atomic spectrometry is a powerful method of analysis. It consists of introducing a liquid sample into means of treatment coupled with means of spectrometric analysis to produce spectra representing the bodies contained in the sample. Four forms of analytical atomic spectrometry are known: Atomic Absorption Spectrometry or AAS; Atomic Emission Spectrometry or AES; atomic fluorescence, and Mass Spectrometry or MS.
Traditionally, atomic absorption consists of introducing the solution into a flame at very high temperature, conventionally in the region of 2000 to 3000.degree. C., lit by a lamp corresponding to the bodies to be analysed. This lamp is, for example, a hollow cathode lamp and usually allows the analysis of one to three elements, such as, for example, mercury or chromium. The light emitted by the lamp after crossing through the flame is subjected to spectrometric means which show absorption due to the solution ionised in the flame.
One of the most widely used atomic emission techniques is Inductively Coupled Plasma atomic emission spectrometry--ICP-AES.
Similarly, a technique frequently used for mass spectrometry is the ICP technique, or ICP-MS.
For all analytical atomic spectrometry techniques, the means of introducing the sample is of determinant importance for the success of analysis. But, it is often necessary to obtain a very low concentration detection limit. In particular, analysis of toxic elements in water requires very high sensitivity in order to reach promulgated, authorised limits.
One of the most effective means of introducing the sample consists of previous nebulisation with a nebuliser. With nebulisation it is possible to spray the sample into fine droplets so that the energy used by the analytical atomic spectrometry means, aside from that required for ionisation, is substantially reduced.
The nebuliser that is conventionally used is a pneumatic nebuliser, comprising an inlet for the liquid to be analysed, an inlet for a vector gas, and an expelling outlet for fine droplets in suspension in the vector gas derived from spraying of the liquid. The pneumatic nebuliser usually leads to a spray chamber where the largest droplets can be removed which will improve the stability of the spectrum emission. The spray chamber comprises an aerosol outlet directed towards the analytical atomic spectrum spectrometry means, and a drain allowing the evacuation in liquid form of the large droplets not included in the aerosol. In these pneumatic nebulisers, only a small part of the drops containing the bodies to be analysed is effectively directed towards the spectrometry means representing approximately 5% of the solution introduced into the nebuliser. Examples of pneumatic nebulisers and spray chambers can be found in the document published by Barry L. Sharp &lt;&lt;Pneumatic nebulisers and spray chambers for inductively coupled plasma spectrometry--A Review&gt;&gt;, Journal of Analytical Atomic Spectrometry, vol.3, 1988, pp.613-652.
In order to increase the sensitivity of analytical atomic spectrometry systems, ultrasound nebulisers have been suggested. The latter spray the solutions to be analysed in generally more efficient manner than pneumatic nebulisers allowing a gain of approximately a factor of 10 in the signal to noise ratio. Despite this gain, the detection limits for a certain number of elements, in particular mercury, do not reach required limits.
Another technique for introducing samples is the generation of hydrides. This consists of mixing the solution to be analysed with a reagent chosen to produce volatile hydrides formed from certain elements contained in the solution. The hydrides thus generated are introduced directly in gas form into the analytic atomic spectrometry means. This technique, which operates for certain specific elements, leads to a higher level of performance than that of ultrasound nebulisers and allows very low detection limits to be reached. However, it requires complex handling operations and raise stability problems of the hydrides obtained.
Systems have been suggested which use both hydride generation and nebulisation. Jacques Borgnon and Jean-Louis Cadet, in &lt;&lt;Analyse des elements Hg, Se, As, Sn, Sb et Bi en vapeur froide et hydrures par spectrometrie d'emission ICP&gt;&gt;, Analusis, Vol. 16, num.4, 1988, pp. 77-80, suggest a system comprising a multi-pathway peristaltic pump leading a solution to be analysed and an alkaline solution of borohydride towards a reactor intended to mix the latter, a mixing duct leaving the reactor and passing through a coil designed to increase the reaction time, and a bubbling-decanting jar at the end of this duct for gas-liquid separation and removal of the latter. The gas leaving the jar is led to a plasma torch via vector argon. The presence of the borohydride solution achieves a sensitivity gain of between 50 and 100 in comparison with direct spraying. However, the nebulisation of hydrides raises stability-related problems.
The abstract of Japanese Patent JP-01170840 describes a nebulisation system comprising a spray chamber and a nebuliser introduced into the chamber, and a drain tube extended by a U-shaped tube. A reducing agent is introduced into the drain tube in such manner as to generate hydrides.
This system makes control of reaction volume difficult, causes loss of generated gas and leads to pollution between successively introduced samples.